Methods for the guidance of surgical treatment of cancer utilizing handheld beta-sensitive probes in conjunction with tumor-avid radiopharmaceuticals (such as FDG) have previously been developed. These technologies could also be used to assist in the minimally-invasive endoscopic detection and treatment of cancer. The goal of this project is the development of a miniaturized beta-sensitive probe system for use with endoscopes. This system (called Endoprobe) will have three major subsystems: beta-detector, position-tracker and user interface. The beta detection unit will consist of two stacked surface barrier detectors; the first device detects beta particles and photons, while the rear detector only detects photons (due to the shielding effects of the front detector). Thus, the rear detector's signal can be used to correct the signal from the front detector to produce a virtually uncontaminated measure of beta flux. This design has been successfully employed in handheld beta probe systems. The detectors will be housed with a position-tracking sensor inside a sealed, sterilizable, unit that can be attached to the tip of an endoscope. The position-tracking system will facilitate real-time monitoring of the unit's location. Information from the beta det4ector and tracking system, in addition to the video signal from a standard endoscope will be input to a computer workstation for presentation for presentation to the endoscopist. Thus, the third subsystem is the user interface that will be used acquire data from the three sources (detector, tracker and video camera), combine the information and display the integrated image on a computer monitor. In the final stage of the project, the Endoprobe system will be mated with an endoscope and tested in a series of simulated searches for esophageal cancer using realistic phantoms. The search for esophageal cancer was chosen because this type of cancer has been reported to have good affinity for FDG, and because esophageal cancer is a significant health risk which can be effectively treated with minimally-invasive techniques it is detected in early stages. Accumulation of FDG in the normal organs of the abdomen will be simulated by an anthropomorphic torso phantom filled with the appropriate amounts of FDG. The normal esophagus will be simulated by a flexible tube soaked in FDG. Esophageal lesions (both benign and malignant) will be simulated by thin disks of gelatin containing the appropriate amounts of FDG. The phantoms will be heated to simulated body temperature so that the effects of elevated ambient temperature on the operating characteristics of the detector unit can be assessed. A physician experienced in performing esophageal endoscopies will employ the Endoprobe system in examinations of the simulated esophagus. The results from these tests will be used to assess the effectiveness of the system and techniques for it use in detecting esophageal cancer, and evaluated the desirability of applying Endoprobe to other endoscopic procedures.